A lithographic apparatus is a machine that applies a desired pattern onto a substrate. The lithographic apparatus is used in manufacturing integrated circuits (ICs). A patterning device, which is alternatively referred to as a mask or a reticle, contains a circuit pattern to be formed on an individual layer of the IC. The circuit pattern is transferred onto a target portion of a silicon wafer substrate by illuminating the patterning device with radiation. The lithographic apparatus transfers the circuit pattern by exposing the image onto a layer of radiation-sensitive material (resist) on the wafer. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate. In general, a single substrate contains an array of adjacent target portions that are successively patterned.
It has been proposed to immerse the substrate in an immersion fluid having a high refractive index relative to that of air, to fill a space between a final element of the projection system and the substrate. The point of this is to allow imaging of smaller features since the exposure radiation will have a shorter wavelength in the fluid. The effect of the immersion fluid may also be regarded as increasing both an effective numerical aperture (NA) of the system and depth of focus.